Many physical effects have been considered for use in non-volatile memories. One such effect is creation of conductive filaments in an otherwise insulating oxide material. These conductive filaments are formed by oxygen vacancies, and result in a variable resistance between two electrodes sandwiching the oxide material. In order to create a variable resistance for these conducting filaments, one of the electrodes needs to be able to take up oxygen from the oxide (to provide the low resistance state) and release it back to the oxide (to provide the high resistance state). These changes in resistance are the relevant state changes for information storage. It is convenient to refer to the electrode that takes up and releases oxygen in this way as the SET electrode. Prior to normal operation of such devices, a ‘forming’ step is typically performed by applying a relatively high voltage (i.e., higher than normal operating voltages for changing state) to the device. This initial forming step determines which of the two electrodes is to be the SET electrode according to the forming voltage polarity. More specifically, the SET electrode will be biased positive relative to the RESET electrode in the forming step. Such devices are often referred to as RRAM (resistive RAM) devices.
One recently considered device structure for RRAMs is shown on FIG. 1A. Here 102 is a TiN electrode, 104 is an oxide (hafnium oxide), and 110 is a 2-D graphene electrode sandwiched between insulators 106 and 108. In this work, the device is formed such that TiN electrode 102 is the SET electrode, leading to device operation as shown on FIG. 1B (low resistance state) and FIG. 1C (high resistance state), where the resistance is affected by the presence (or absence) of oxygen (gray circles) in filament 112. Further details relating to this work can be found in (Sohn et al., ‘Atomically thin graphene plane electrode for 3-D RRAM’ IEDM 2014, 5.3.1 to 5.3.4), hereby incorporated by reference in its entirety.